The present invention relates in general to high temperature, high pressure sensors for corrosive liquid and gaseous fluids, and in particular to a new and useful metal sealing system for high pressure, high temperature and corrosion resistant ceramic/sapphire based optical diaphragm sensors for use in harsh applications. The invention describes solutions of the problems associated with these requirements particularly the leak tight and corrosion resistant sealing of the sensor cell to the casing, the sealing of the reference chamber of the sensor and the sealing stress relief for achieving long-term stability of the sensor. This technology can also be used for pressure and vacuum sensor applications using optical, electrical or other means of measuring diaphragm deflection, where particularly corrosion resistance is required.
The technical field of the invention includes high temperature, high pressure sensors for corrosive liquid and gaseous fluids, packaging of sapphire/ceramic sensors in metal casings, metal sealing systems, fiber-optic low-coherence interferometry, capacitive deflection measurement, pressure and vacuum sensor applications and especially for oilwell downhole or drilling applications.
Due to the expected shortage of oil in oil reservoirs, high pressure is building up on new offshore sub sea developments as well as on accessing other reservoirs such as oil sands. Therefore new technologies are needed for recovering oil, including pressure and temperature measurement. Pressure measurement will provide a better control of the oil extraction process. It is expected that better management of reservoirs can increase the share of recoverable oil by 10% to 20%.
Pressure measurement is a crucial part of new extraction technologies, particularly Steam Assisted Gravity Drainage (SAGD), for recovery of oil from vast reservoirs of oil sands. Major deposits are located in Canada, Venezuela, United States, Russia and the Middle East.
All those factors contribute to the requirement for new technologies for pressure measurement in increasingly deeper wells, which goes along with higher temperatures and thus more chemically aggressive environments. These are the drivers for new technologies for thermal and chemical resistant high pressure and high temperature sensors, which need to work reliably in several kilometers depth in the 1000 bar range and at several 100° C. Conventional sensors with integrated electronics can no longer work in such demanding environment, as they work only up to the 180° C. range according to their specifications.
A fiber optic system is generally considered to be a viable solution as the completely passive optical sensor can be designed and packaged to fulfill the specific operating conditions and the optical signal then can be transmitted over long distances without loss of signal quality. In addition, since no electrical signals are transmitted, the system is free of EMI problems and is intrinsically safe.
Optical diaphragm gauges have been described in the literature (e.g. Lopez-Higuera, 2002; Totsu et al., 2003). Such an instrument is basically a diaphragm gauge. The readout of the gauge is done by optical means. There are many optical techniques available to measure the distance between two parts. However in practical pressure measurement where distances in the range of a tenth of an Angstrom to a millimeter must be measured, mainly Fabry-Perot principles are used. Primary applications have been chemical process monitoring and biomedical applications. These sensors are typically operated at pressures above atmospheric pressures. Optical methods for the measurement of the membrane displacement at temperatures up to 550° C. have been realized in some commercial products like Luna Innovations' Fiber Optic Pressure Sensor using external Fabry-Perot Interferometry. Sensors by Taitech, FISO Technologies or Davidson Instruments, use Silicon MEMS technology. Virginia Tech has constructed a single-crystal sapphire sensor.
A typical packaging method to attach sapphire/ceramic sensors to a metal housing, either uses elastomer o-ring sealing or brazing. Elastomer sealing systems can not be used for high temperature applications above 150° C. and depending on the elastomer type, also not in conjunction with corrosive media. Brazing leads to permanent bonding and sensors therefore can not be exchanged easily. Furthermore, typical brazing solutions are prone to corrosive attacks and to thermal stress due to mismatch of thermal expansion coefficients of the involved materials, e.g. Vacon, usually used as intermediate material, is not corrosion resistant at elevated temperatures and the corrosive media expected in oilwell downhole applications.
European Patent EP 0 461 459 B1, filed on May 28, 1991 (corresponding to U.S. Pat. No. 5,174,157), describes the sealing of a ceramic pressure cell with a sealing ring consisting of a fluoroelastomer that seals on a glass layer applied on the outer membrane section of the ceramic sensor diaphragm. Such a sealing system can be used in ambient temperature environment but can not be used in high temperature applications above 200° C. and with corrosive media since fluoroelastomers are generally not suitable or not accepted for corrosive applications. Thus, for high temperature and corrosive media applications a metal sealing system would be preferred. Metal seals are generally harder than elastomers and thus one would not expect and it is not obvious such glass coatings to resist the high sealing forces and point loads generated by the hard metal seals. If the hard seal is directly applied on the diaphragm portion of the sensor, then this leads to stress in the sensor which results in initial bending of the sensor and stress relief over time generally recognized as drift of the sensor. In order to avoid such sealing stress on the diaphragm the sealing section of the sensor is not directly on the diaphragm. In cases were the glass layer is not resistant to corrosive media the glass layer needs to be coated with a protective layer of ceramic or other suitable materials.
European Patent EP 0 372 988 B1, filed on Dec. 8, 1989 (corresponding to U.S. Pat. No. 4,888,662) describes the sealing of a capacitive ceramic sensor cell for high pressure applications with o-ring seals of different materials on the diaphragm portion of the sensor with materials softer than polytetrafluoroethylene (Teflon) and includes also metallic materials. Soft metallic materials have the advantage of being deformed by the sealing forces and thus being able to properly seal on the ceramics. The disadvantages of these ductile materials are their low melting points, which makes them not suitable in high temperature applications. Furthermore, they are not resilient in case of gap variations due to thermal mismatch of the sealing system members or due to the pressure applied on the sensor, and in many cases they are not suitable for corrosive applications. Additionally the seal is directly applied on the diaphragm portion of the sensor which leads to stress in the sensor that result in initial bending of the sensor and stress relief over time generally recognized as unwanted drift of the sensor.
Problems/Disadvantages/Deficiencies:
Various pressure-temperature sensors with different technologies are presently used in oilwell downhole applications, among which the major technologies are strain gauges and vibrating wire gauges. Most are limited to temperature ranges compatible with electronics, which is specified to below 200° C., in most cases to 177° C. Based on information from companies in the oil recovery market, sufficient performance for permanent downhole sensors can only be expected up to the 120° C. range, rather than in the specified range. Optical sensors are widely recognized to potentially solve the temperature problem related to the electronics, since no electronics are exposed to high temperature.
The main problem in manufacturing such an optical sensor is to make it withstand the extreme conditions while being stable, sensitive, resistant to corrosive fluids and gases, long-lasting and to be able to reliably connect the signal-transferring fiber-optic cable to the sensor.
An attempt to make a complete all single-crystal sapphire sensor has been made for example in U.S. Patent Application US 2005/0195402 A1. This solution is restricted to a small membrane size, leading to reduced sensitivity, poor reference pressure quality, leading to hysteresis effects and temperature dependence, and to a structure which is difficult to combine with other materials in a robust and leak-tight fashion.
One of the main problems when manufacturing a practical optical sensor for the described purposes, is the mounting of the optical sensor cell to the surrounding instrument chassis, which are usually made of industrial standard metallic alloys. One requires a solution where the mounting would be robust, leak-tight, corrosion resistant, long-lasting, stable and most importantly such that it has a minimal effect on the sensor performance in high temperature and high pressure environment.
The typical sealing method for ceramic high pressure sensors is using polymer o-rings that are sealing directly on the diaphragm. Using sealing rings, such as o-rings, for clamping directly on the diaphragm can easily have a big effect on the signal via non-desired bending of the diaphragm, which moreover can well be temperature dependent. This causes unwanted response and drift of the sensor, which is above the accuracy and long-term stability requirements for oilwell downhole applications and other applications that have similar requirements. Direct clamping of the diaphragm with the level of force required for the sealing of several hundreds of bar of pressure is certain to cause unevenly distributed load across the diaphragm.
One of the main problems in combining different materials, for example ceramic and metal, is the different Coefficient of Thermal Expansion, causing expansion mismatches between parts and thus stress gradients in the structure. Moreover, a high level of stress tends to relax one way or another, for example by creep effects such as movement of dislocations or viscous flow, appearing as long-time drift in the measurement signal.
Polymer based sealing materials are typically not suitable for temperatures above 200° C. and pressures above 20 bar. Metal based sealing materials in combination with sapphire based sensors require high grade of polished surfaces and suffer from sensor cracking under uneven sealing pressure distribution and high point loads. Using ceramic-based sealing surfaces, e.g. alumina instead of sapphire, require soft sealing materials due to the remaining roughness of the polished surface caused by voids in the polycrystalline surface structure (break outs of single grains). In addition polishing increases the probability of cracking which requires additional annealing processes and thus increases cost. The disadvantage of soft sealing materials is their incompatibility with the high temperature, high pressure, corrosive environment and the insufficient elasticity to compensate for minute changes caused by the mismatch in thermal expansion coefficients of the materials involved.